Nitride-based LEDs using p-n junction of AlN, GaN, InN which are nitrides of the Group XIII and the mixed crystal phase thereof have been widely put to practical use. Since the nitride-based LEDs are inorganic substances, and have high binding energy of materials, it is known that they have a long lifetime, and have a high internal luminescent efficiency (internal quantum efficiency) of 90%. These nitride-based LEDs are often mass-produced on a high-priced monocrystalline substrate, such as sapphire or silicon carbide, using a metallo organic chemical vapor deposition method (MOCVD method) having low mass productivity. For this reason, they are expensive for use as a surface light source, and thus have been used only as a point light source.
On the other hand, organic EL devices are known as a surface light sources (see, for example, Patent Document 1). Since the organic EL devices can use a low-priced plastic substrate or glass substrate as a starting material, the prices of the devices can be lowered, and use as a surface light source is possible. In addition, use as a bendable light emitting device or illumination is also anticipated.
[Patent Document 1] Japanese Unexamined Patent Publication No. 2008-21480
However, since a luminescent layer included in the organic EL is an organic substance, there has been a problem in that it has low heat resistance and a short lifetime. In addition, it has a lower luminescent efficiency than that of the nitride-based LED.